feat: make sandbox creation timeout configurable

[guyoron1]

Summary

Make sandbox creation and readiness timeouts configurable via environment variables instead of hardcoded constants.

• FULLSEND_SANDBOX_TIMEOUT — overrides the 65s creation timeout
• FULLSEND_SANDBOX_READY_TIMEOUT — overrides the 60s readiness timeout

Fixes #4

Changes

• Added resolveTimeout() helper in internal/sandbox/sandbox.go
• Renamed constants to defaultCreateTimeout / defaultReadyTimeout
• Timeouts cascade: env var > default

Test Plan

• Verify default behavior unchanged (no env vars set)
• Verify custom timeout via FULLSEND_SANDBOX_TIMEOUT=120s
• Verify invalid duration strings fall back to default
• Unit tests for resolveTimeout cascade logic

[guyoron1]

/fs quality

[guyoron1]

/fs quality

[guyoron1]

/fs quality

[github-actions]

QualityFlow Pipeline Summary

Stage	Agent	Status
1	STP Builder	✅
2	STP Reviewer	✅
3	STP Refiner	✅
4	STD Builder	✅
5	STD Reviewer	✅
6	STD Refiner	✅
7	Go Test Generator	✅
8	Python Test Generator	✅

Test Output

Type	Count	Location
Go/Ginkgo tier 1	3 files	qf-tests/GH-4/go/
Python/pytest tier 2	4 files	qf-tests/GH-4/python/

Issue: GH-4

---

Generated by QualityFlow

[guyoron1]

/fs quality

[guyoron1]

/fs quality